ESCON is a standard computer system interface and protocol developed by International Business Machines (IBM). ESCON is applicable to an environment such as the one shown in FIG. 1, in which a central processing unit (CPU) 1 is coupled to a channel 3 having direct access to a main memory (not shown) of the CPU. The channel provides input/output capability to the CPU. More particularly, the channel 3 is coupled, over a link 5, to one or more control units 7, which each is in turn coupled to a plurality of I/O peripherals 9. Control unit 7 controls access to CPU 1, via channel 3, for each of the I/O peripherals 9, allowing each to have direct access to the CPU memory.
ESCON defines the interface and protocol for communicating over link 5 between channel 3 and control unit 7, and is described in detail, for example, in "ENTERPRISE SYSTEMS ARCHITECTURE/390--ESCON I/O INTERFACE", Third Edition (1992) published by IBM, which is incorporated herein by reference. Dedicated ESCON links have been provided so that control unit 7, along with its associated I/O peripherals 9, can be disposed at a location somewhat remote from CPU 1 and channel 3. However, these dedicated links have conventionally been limited to relatively short distances. For example, IBM states that its dedicated ESCON links are limited to approximately 60 km. Although this number may be conservative, it is believed that conventional dedicated ESCON links are limited to distances of approximately 80 km, even when appropriate repeaters are used along the line.
Conventional dedicated ESCON links between a channel and control unit suffer from two significant disadvantages. First, the above-described limit on the maximum supported distance may be insufficient for some applications. Second, it is often costly to implement these dedicated links because they are not part of a public communication network, and may not be capable of being implemented over pre-existing communication lines.
It is an object of the present invention to provide an improved interface for enabling communication between two remotely disposed devices, including ESCON compatible devices.